In optical transmission networks, in the event of disturbances to a connection, a so-called protection connection is switched via another transmission path. For ring networks, instances of such standby switching are described, for example, in “26th European Conference On Optical Communication ECOC 96, Oslo, pages 3.51 to 3.54”. In the so-called “hot standby” mode, the same message is continuously transmitted in parallel via additional protection data paths, so that a switchover can be effected without a time delay in the event of a disturbance.
Additional problems also arise in the context of optical transmission by means of wavelength division multiplex technology, which problems make it more difficult to identify a line breakage or other failure of the useful signal. For safety reasons, the optical transmission signal is switched off in the event of a disturbed connection; for example, by the laser or an optical amplifier connected downstream being switched off or by a switch being actuated. These measures are referred to as laser safety switch-off.
It is thus customary to transmit further data in addition to the useful data in a service channel, so that even in the event of the failure of useful data (payload), a signal with a non-negligible level is received. On the other hand, the transmission signal or transmission channel should not be switched off even when no useful signal is transmitted or, in the case of a wavelength division multiplex signal, no channel is busy.
An object of the present invention is to determine a reliable criterion for the failure of the useful signal and, in this case, if possible, to generate a signal for fiber standby switching but to avoid instances of erroneous standby switching. Furthermore, the intention is to perform laser safety switch-off of the disturbed transmission line which enables renewed switching-on without any problems.